The use of cavity die molds has progressed from the use of single cavity dies, through multi-cavity dies, to the use of 2-level stacked mold arrangements. In such arrangements, the available axial compressive force provided by the molding press is utilized in a pair of molds arranged in back-to-back relation, so as to effectively double the productive capacity of the machine for substantially the same press loading. Such an arrangement gives a doubled production rate with little increase in the axial loading required of the press.
The production levels that are presently achievable are limited to 2-level stack molding i.e. the number of articles per injection cycle is limited to the total contents of each of two levels, being generally twice the number of cavities per single mold face or level. One sandwich molding cycle (Sorensen) has 4-levels of molding cavities. However, the injection cycle is effectively divided into two sequential cycles, involving re-clamping of the press platen and separate cooling and ejection cycles.
The effective working of such arrangements, depends among other things, on the provision of balanced feeder flow paths to all of the die cavities.
The mold stack is subject to differential expansion, axially, between different groups of parts thereof, namely the hot runner system and the mold body.
Various aspects of the prior art relating to injection molding, including die construction for stock flow symmetry; inflow feed nozzles; valve gate actuation; melt or feed transfer, including mold block to mold block stock transfer; drool prevention; mold stack construction; and block heater provisions may be found in the following U.S. patents, cited as being but illustrative of the prior art: 2,770,011 Nov. 1956, Vely; 3,533,594 Oct. 1970, Segmuler; 3,806,295 Apr. 1974, Gellert; 3,843,294 Oct. 1974, Bielfeldt et al.; 4,207,051 June 1980, Wright et al.; 4,212,626, July 1980, Gellert; 4,309,163 Jan. 1982, Cottancin; 4,473,347 Sept. 1984, Terashima; 4,477,242 Oct. 1984, Eichlseder et al.; 4,5389,171 Sep. 1985; 4,586,887 May 1986, Gellert; 4,669,064 July 1986, Landis et al.; 4,663,811 May 1987, Gellert; 4,559,971 June 1987, Gellert.